Field of the Invention
The present invention relates to a technology of a reading apparatus configured to read image information that is recorded on an original.
Description of the Related Art
An image reading apparatus used in copying machines, facsimile machines, and other image forming apparatus is demanded to have, among others, a high throughput in reading an original and a small risk of damaging or tearing an original that is being conveyed by avoiding making a conveyance path complicated. A reading apparatus that has been developed to meet such demands is capable of reading images on the front side and back side of an original at one time.
For instance, an image reading apparatus disclosed in Japanese Patent Application Laid-open No. 2002-335380 employs a configuration in which a front-side reading unit and a back-side reading unit are opposed to each other across a short distance in order to downsize the apparatus.
Specifically, a white reference board used to correct an optical system of the back-side reading unit is provided on a light transmissive platen on the side where the front-side reading unit is placed, in a place where the path of light used by the front-side reading unit to optically read an original is not obstructed. This keeps the white reference board from staining.
However, the white reference board of the back-side reading unit in Japanese Patent Application Laid-open No. 2002-335380 is positioned farther than a reading point. A characteristic of light read at a point far from the light source is that the distribution of the light is wide in a dark range.
An example is discussed in which shading correction is executed by reading the white reference board at a long distance from the reading point and generating shading data for shading correction.
In this case, there is luminance unevenness in which the luminance at left and right edge portions (main scanning left and right edge portions) of a reading range in a main scanning direction, which is a direction orthogonal to an original conveying direction, is lower than in a central portion of the reading range (main scanning central portion). The resultant problem is that a shading data value for the main scanning left and right edge portions is relatively higher than a shading data value for the main scanning central portion.
Another problem is in that, when an original is read while allowing the shading data value at the main scanning left and right edge portions to remain high, the luminance is higher at the main scanning left and right edge portions than in the main scanning central portion because the reading point is closer than the white reference board. The influence of the luminance at the main scanning left and right edge portions of the reading range can be eliminated by placing a light source that runs down a long stretch in the main scanning direction. A problem in this case is in that the large size of the reading unit makes the overall size of the apparatus larger.
It is therefore a main object of the present invention to provide a reading apparatus capable of preventing luminance unevenness in the reading range by correcting shading data. The present invention further provides an image forming apparatus having a function of the image reading apparatus and an image forming system having the function of the image reading apparatus.